This invention relates to gas-fired infra-red burners such as those used in ovens and broilers.
Gas-fired infra-red burners are widely used in both commercial and domestic cooking ranges. Such gas ranges typically have a broiler unit mounted at the top surface of an oven and oriented downwardly to provide a top-broil feature. A burner unit generally comprises a plenum chamber supplied with a combustible gas/air mixture through a venturi and a burner element made from either perforated ceramic tile or a wire mesh through which the gas/air mixture escapes. The gas/air mixture is ignited to produce a flame adjacent the outer surface of the tile or mesh.
A metal mesh screen is often preferred to a ceramic burner for durability reasons. A metal screen, however, undergoes significant thermal expansion and contraction during use as it cycles between room temperature and approximately 1500xc2x0 F. When a flat piece of screen is secured to the burner plenum around its edges, this thermal expansion can cause warping, rippling, and other undesirable deformation of the screen. Some prior art burners feature a screen formed with a series of ridges or undulations prior to being secured to the plenum, the ridges absorbing the dimensional changes from expansion and contraction by varying somewhat in height and/or horizontal spacing.
In general, a large burner area is preferable in order to provide relatively even heating over the surface of an object or objects being cooked, hereinafter known as the load. A burner, however, always heats the portion of the load adjacent the center of the burner more than the portion away from its center. This is because every point on the burner radiates heat to all points on the load along a line-of-sight, with the intensity of the heat transferred to the load being inversely proportional to the distance between the burner and the load. Consequently, the load adjacent the center of the burner receives intense infra-red rays traveling the relatively short distance from the center of the burner, plus relatively weaker rays traveling the greater distance from the edges of the burner.
The portions of the load adjacent the edges of the burner, however, receive only a fraction of the total heat received by the central portion, since these edge portions are not completely surrounded by the burner.
It is known to create a burner having a more even heat distribution by providing a non-radiating zone at the center of the burner. This has been achieved in ceramic tile burners by placing a metal blocking plate on the rear surface of the tile and sealing the periphery to the plate to the tile with a gasket. This blocks the flow of gas through the holes in the center of the tile so that no flame is present in the central area of the burner.
In the past it has been impractical to construct a wire mesh burner in which the gas flow is blocked in an area at the center of the burner to create a non-radiating zone. The primary problem is due to the difficulty in securing a metal plate to a wire mesh in a manner that will survive when the burner expands and contracts due to heating and cooling during use. A wire mesh screen and a solid metal plate undergo thermal expansion of different magnitude, at different rates, and in different directions. This expansion differential makes it extremely difficult to secure the plate to the mesh in a manner that will remain intact and not result in rippling and/or other undesirable deformations of the wire mesh and attached blocking plate during the many cycles of thermal expansion and contraction that occur during normal use. This problem is particularly pronounced when the area of the burner is large.
According to the present invention, a gas-fired infra-red burner comprises a plenum for receiving a gas/air mixture and having a generally planar burner opening, and a wire mesh screen secured to the plenum around the perimeter thereof and bulging out of the plane of the burner opening away from the plenum. This bulge or dome effect allows the screen to undergo thermal expansion when the burner is operating without any rippling, waviness or other undesirable deformation of the screen. Rather, expansion of the screen causes the dome to increase slightly in height in a uniform, controlled manner.
According to a further feature of the invention, a metal plate is secured to the approximate center of the screen to block the flow of gas therethrough, thus creating a non-radiating zone at the center of the burner. Such a burner provides a more even heat so that portions of the load adjacent the center of the burner do not receive substantially more infra-red heating than those portions closer to the edges of the burner.
In the preferred embodiment of the invention, the blocking plate is formed with a plurality of creases radiating from the approximate center of the plate in a star-shaped pattern, and is bent about at least one of the creases so that the plate has a concave surface which contacts the screen. The plate is spot welded to the screen at a plurality of points adjacent the edges of the plate. As the plate expands when heated, the creases grow in height so that the overall horizontal dimensions of the plate do not increase as much as would a flat plate. Accordingly, the creases allow the plate to expand in a controlled manner so that the plate and the wire screen do not shift or separate relative to one another during temperature changes.
According to another feature of the invention, the venturi tube which feeds the gas/air mixture to the burner has a circular cross section adjacent its upstream end and transitions to a flattened, elliptical cross section adjacent the end where it connects to the plenum. The major axis of the elliptical cross section is parallel to the burner opening so that the depth of the plenum, and hence the overall depth of the entire burner unit, may be smaller than if the circular cross section were connected directly to the plenum. The internal area of the elliptical section be at least equal to the internal area of the circular section at its largest diameter to avoid a constriction that would interfere with proper flow of the gas/air mixture into the plenum.
According to yet another feature of the present invention, the dome shape of the screen is achieved by bending portions of the screen adjacent parallel, opposite edges of the screen in the direction of the desired bulge to form tabs which are then secured to corresponding parallel, opposite edges of the plenum. This causes the central portion of the screen to bulge outwardly from the plenum.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.